This invention relates generally to the decompression of electronically transmitted and stored images and specifically to a method of eliminating ringing artifacts occurring with some image compression methods under medium and high compression ratios.
A color image acquired by current digital cameras may have the equivalent of 100 pages of text information. Higher quality images from next generation xe2x80x9cmegapixelxe2x80x9d cameras exceed the capacity of common floppy disk storage and strain voice grade telephone communication channels.
A variety of data compression techniques are known to reduce the amount of image data that must be stored or transmitted. Highest compression ratios are obtained by xe2x80x9clossyxe2x80x9d compression schemes where compressed data is irreversibly degraded, for example, by using larger pixels or fewer gray levels or colors, or by more sophisticated techniques which truncate spatial frequency information. Sophisticated lossy compression schemes such as JPEG and MPEG attempt to discard information that is not critical to the perception of a typical human viewer. These systems take advantage of known information about the human visual system (xe2x80x9cHVSxe2x80x9d).
Lossy compression schemes may be combined with xe2x80x9closs-lessxe2x80x9d compression schemes where the data is compressed without loss of information, for example, through xe2x80x9czero length encodingxe2x80x9d in which a string consisting of a number of consecutive zeros in the image, or more generally any pattern of consecutive pixels, is replaced with a shorter code designating that number or pattern.
After the image is compressed it may be decompressed by a program which generally restores the compressed and encoded data into a human readable format. With high compression ratios, and lossy compression systems, image artifacts may appear in the decompressed image. One such, artifact is ringing, in which spurious ripples flank the edges of abrupt contrast changes. Ringing artifacts result from a loss of high spatial frequency information necessary to accurately represent the edge. The human visual system is known to be sensitive to ringing artifacts which practically place an upper limit on the amount of useful compression of electronic images.
The present invention provides a method and apparatus for post decompression reduction of ringing artifacts. Generally, the invention identifies edges in the decompressed image and then, based on those edges, defines zones about the edges where ringing artifact may be prominent. These zones may be modified based on an a priori modeling of the human visual system and then the image within these zones is filtered to reduce the ringing artifacts. The definition of the zones is such as to exclude the edges themselves and to minimize filtering in areas where the ringing would not be perceived.
Specifically, the present invention provides an image processing system receiving a decompressed image and having an edge detector identifying edges between contrasting regions of pixels of the image. A mask generator working with the identified edges defines a region in the image adjacent to and conforming to the identified edges. A low pass spatial filter operating only within the defined regions filters the decompressed image to selectively reduce ringing artifacts near those edges.
It is therefore one object of the invention to permit increased compression of images by reducing ringing artifacts. The selective identification of zones for filtering decreases the level of the ringing artifacts while preserving edge structure and other features of the image.
It is another object of the invention to provide artifact reduction for a variety of image compression techniques without the need to modify the compression or decompression techniques or to necessarily have knowledge of the particular compression technique being used. The invention, in its essential form, works directly and only on the decompressed image. As a result, application of the invention is not limited to current image compression techniques but may be applicable to future image compression and decompression methods in which ringing artifacts is a problem.
The image processing system of the present invention may include a model of the human visual system manifest as one or more properties from which rules are derived which are used to modify application of the low pass filter according to a perceptional model of the sensitivity of standard human vision. For example, the human visual system model may reduce the need for low pass filtering of regions that have low brightness values or high variance in brightness values as is determined from the decompressed image. The modification may be done by modifying the regions to which the filter is applied.
Thus it is another object of the invention to minimize filtering, and thus the risk of unnecessary image degradation, in portions of the image where ringing artifacts would not be objectionable to a human viewer.
The detection of the image edges and the defining of the mask regions as well as the low pass spatial filtering may be performed by binary and gray scale morphological operators.
Thus it is another object of the invention to provide a method of reducing ringing artifact that requires only simple binary or integer arithmetic and thus which may be performed at high speed in specialized electronic circuits or on a computer processor.